feat(mstsgu): add RPC integrity framing

[Marc-André Moreau (mamoreau-devolutions)]

Provide structural packet-integrity DCE/RPC framing with explicit caller-owned signed regions and verifier validation.

Reject unsupported authentication levels and clear terminal response state after failed decoding. Verifier construction, sequence ownership, and RPCH transport remain with a future security context.

[github-actions]

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[Copilot]

Pull request overview

Adds caller-managed DCE/RPC packet-integrity framing to the MSTSGU codec layer.

Changes:

• Adds authenticated PDU preparation, decoding, and response reassembly.
• Adds validation and comprehensive framing tests.
• Updates crate capability documentation.

Review scope: prose-verifier was skipped because the prose change is a single factual list item.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
crates/ironrdp-mstsgu/src/rpc.rs	Implements integrity framing and authenticated reassembly.
crates/ironrdp-mstsgu/tests/rpc_pdu.rs	Tests authenticated framing, validation, and recovery.
crates/ironrdp-mstsgu/README.md	Documents packet-integrity support.